Source : Webster's Revised Unabridged Dictionary (1913)

I O U \I O U\ [i. e., I owe you.]
   A paper having on it these letters, with a sum named, and
   duly signed; -- in use in England as an acknowledgment of a
   debt, and taken as evidence thereof, but not amounting to a
   promissory note; a due bill. --Wharton. Story.